The present invention relates to a system for regenerating depollution means associated with oxidation catalyst-forming means integrated in an exhaust line of a motor vehicle diesel engine.
More particularly, the invention relates to a system in which the engine is associated with common manifold or “rail” means for feeding fuel to the cylinders of the engine and adapted, at constant torque, to implement a strategy of regeneration by injecting fuel into the cylinders, in at least one post-injection operation.
During regeneration of depollution means such as, for example, a particle filter, stages during which the engine is idling (very low exhaust temperature) or during which the vehicle accelerator pedal is being raised (no injection of fuel in normal operation), are problematic since they cause a drop in the temperature of the exhaust, i.e. the exhaust line and the elements integrated therein.
The use of one or more post-injections during such stages in the operation of the engine makes it possible to limit the drop in the temperature of the exhaust line by relying on the catalytic conversion of the hydrocarbons (HCs) produced by the combustion of the post-injection(s) in the engine.
However, those strategies rely on the exothermic reaction produced by the catalyst-forming means, it being assumed that said means are active, where said means comprise, for example, an oxidation catalyst or a NOx trap with a carbon monoxide/hydrocarbon (CO/HC) oxidation function.
During stages in which the engine is returning to idling as a result of the accelerator pedal being raised, there is no main injection nor any pilot injection, so the or each post-injection does not burn in the cylinder, since post-injection merely causes the fuel to be vaporized in the form of HCs which are subsequently converted by the catalyst-forming means.
The inlet temperature of the oxidation catalyst-forming means is thus very low, and in spite of the exothermic catalytic reaction that is produced by the combustion of the HCs derived from the or each post-injection, the front face of the catalyst-forming means cools down progressively and its conversion activity becomes progressively deactivated.
While the engine is idling, in spite of using one or more post-injections, the temperature at the inlet to the catalyst-forming means is relatively low. The strategy of post-injection while idling also relies on the catalytic conversion of the HCs produced by the combustion of the post-injections into the engine. In spite of the catalytic reaction being exothermic, the front face of the catalyst-forming means cools down progressively and its conversion activity becomes progressively deactivated.
During the stage of returning to idling or a stage of prolonged idling, it can happen that the catalyst-forming means are thus no longer sufficiently active to convert all of the HCs, which leads to peaks of HCs downstream from the catalyst-forming means, and even to blue smoke and/or exhaust odors.
Furthermore, the use of post-injections leads to the lubricating oil being diluted with fuel, thereby degrading its lubrication properties, in particular reducing its viscosity, and if viscosity becomes too low, that can lead to damage to the engine.